Tony is filmed in the shower. As such, some characters make sexual references.
Throughout the series, Matt's therapist talks about his various sexual encounters in explicit detail. It is for comedic effect, but he goes into graphic detail on occasion.
Two characters are seen having sex. No nudity is shown, and it's mainly played for comedic effect.
When characters are having sex, you do not see it and it is rustling under the sheets. However, it only lasts for a couple of seconds.